Activation of oxygen-responsive pathways are associated with altered protein metabolism in Arctic char exposed to hypoxia

Abstract

Fish exposed to fluctuating oxygen concentrations often alter their metabolism and/or behaviour to survive. Hypoxia tolerance is typically associated with the ability to reduce energy demand by supressing metabolic processes such as protein synthesis. Arctic char is amongst the most sensitive salmonid to hypoxia, and typically engage in avoidance behaviour when faced with lack of oxygen. We hypothesized that a sensitive species will still have the ability (albeit reduced) to regulate molecular mechanisms during hypoxia. We investigated the tissue-specific response of protein metabolism during hypoxia. Little is known about protein degradation pathways during hypoxia in fish and we predict that protein degradation pathways are differentially regulated and play a role in the hypoxia response. We also studied the regulation of oxygen-responsive cellular signalling pathways [hypoxia inducible factor (HIF), unfolded protein response (UPR) and mTOR pathways] since most of what we know comes from studies on cancerous mammalian cell lines. Arctic char were exposed to cumulative graded hypoxia trials for 3 h at four air saturation levels (100%, 50%, 30% and 15%). The rate of protein synthesis was measured using a flooding dose technique, whereas protein degradation and signalling pathways were assessed by measuring transcripts and phosphorylation of target proteins. Protein synthesis decreased in all tissues measured (liver, muscle, gill, digestive system) except for the heart. Salmonid hearts have preferential access to oxygen through a well-developed coronary artery, therefore the heart is likely to be the last tissue to become hypoxic. Autophagy markers were upregulated in the liver, whereas protein degradation markers were downregulated in the heart during hypoxia. Further work is needed to determine the effects of a decrease in protein degradation on a hypoxic salmonid heart. Our study showed that protein metabolism in Arctic char is altered in a tissue-specific fashion during graded hypoxia, which is in accordance with the responses of the three major hypoxia-sensitive pathways (HIF, UPR and mTOR). The activation pattern of these pathways and the cellular processes that are under their control varies greatly among tissues, sometimes even going in the opposite direction. This study provides new insights on the effects of hypoxia on protein metabolism. Adjustment of these cellular processes is likely to contribute to shifting the fish phenotype into a more hypoxia-tolerant one, if more than one hypoxia event were to occur. Our results warrant studying these adjustments in fish exposed to long-term and diel cycling hypoxia.

Phenotypic plasticity during diel cycling hypoxia in Arctic char (Salvelinus alpinus)

“…In our study, three hours passed between reoxygenation and sampling; this can probably explain the absence of variation in glucose and lactate between the control and experimental groups. In a previous study on the same Arctic char strain, we detected an increase in muscle lactate after 9 hours of exposure to the same level of hypoxia (Cassidy and Lamarre, 2019). After 3 hours of reoxygenation, accumulated muscle lactate should have been excreted into plasma or remobilized into glycogen, as observed in trout (Milligan and Girard, 1993).…”

“…The rate of hepatic protein synthesis has been shown to curtail during acute hypoxia in Arctic char (Cassidy and Lamarre, 2019). The liver plays a central role in energetic processes, particularly through its involvement in protein metabolism (Charlton, 1996).…”

“…The liver is involved in carbohydrate, lipid, and protein metabolism (Charlton, 1996; Rui, 2014) and as such, plays a central role in energy metabolism. During hypoxia, carbohydrates and lipids are mobilized to produce ATP while the rate of protein synthesis is depressed to reduce energy demand (Cassidy and Lamarre, 2019; Cassidy et al ., 2018; Lardon et al ., 2013; Li et al ., 2018). Moreover, hypoxia may decrease glycogen and lipid stores (Cadiz et al ., 2018; Gracey et al ., 2011).…”

Gilthead Seabream Liver Integrative Proteomics and Metabolomics Analysis Reveals Regulation by Different Prosurvival Pathways in the Metabolic Adaptation to Stress

“…The same was verified for rainbow trout [ 70 ]. The inhibition of the mTOR-signaling pathway was reported in Artic char exposed to hypoxia, precisely 15% of air saturation, along with a decrease in the rate of protein synthesis [ 71 ]. In humans, the dysregulation of the mTOR-signaling pathway has been associated with aging and several diseases [ 72 ].…”

Rapid and reversible modulation of blood haemoglobin content during diel cycles of hypoxia in killifish (Fundulus heteroclitus)

“…Secondly, the higher resting MO2 in hypoxia of the intermittent hypoxia group relative to other treatment groups was also unlikely to have resulted from increased O2 transport capacity, in association with a similar lack of variation in blood haemoglobin content between acclimation groups in hypoxia. The ability of fish in this group to maintain resting MO2 at a higher proportion of MO2,max may instead reflect a change in metabolic regulation during hypoxia to reactivate processes that were initially down-regulated when fish were first exposed to hypoxia, such as protein synthesis (Cassidy et al, 2018;Cassidy and Lamarre, 2019). Such changes in metabolic regulation may therefore be important for maintaining important fitnessrelated functions such as growth and reproduction in fish that are chronically exposed to daily bouts of hypoxia (Bera et al, 2017;Cheek, 2011;Cheek et al, 2009).…”